In a 74.2-GHz 100-kW pulse gyrotron, the characteristics of parasitic low-frequency oscillations and their influence on electron energy spread have been determined. A method for suppressing parasitic oscillations by varying the magnetic field distribution in the beam compression region is studied. The results of measurements demonstrate the influence of azimuthal inhomogeneity of electron emission...
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A simple approximate theory of background plasma influence on the electromagnetic properties of high-power gyrotron cavities is presented. It is shown that a background plasma slightly reduces the operational frequency of gyrotrons, increases the quality factors of operational and neighbouring modes, and modifies the longitudinal field distribution in the gyrotron cavity. The calculated downshift ...
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The operating characteristics of a 140-GHz 14-W long pulse gyrotron are presented. The device is being used in dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP/NMR) spectroscopy experiments. The gyrotron yields 14 W peak power at 139.65 GHz from the TE(0,3) operating mode using a 12.3-kV 25-mA electron beam. Additionally, up to 12 W peak has been observed in the TE(2,3) mode a...
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We report the regulated continuous-wave (CW) operation of a second harmonic gyrotron oscillator at output power levels of over 8 W (12.4 kV and 135 mA beam voltage and current) in the TE0,6,1 mode near 460 GHz. The gyrotron also operates in the second harmonic TE2,6,1 mode at 456 GHz and in the TE2,3,1 fundamental mode at 233 GHz. CW operation was demonstrated for...
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The self-consistent nonlinear simulation of a Ka-band, second harmonic two-stage gyroklystron amplifier is presented in this paper. The beam-wave interaction in the gyroklystron is studied by using a self-consistent nonlinear simulation code GKLSC, and the electron bunching pictures of the different positions in phase space are demonstrated in detail. The effects of various parameters, such as dri...
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The solution of the dispersion relation of a disc-loaded circular waveguide for the propagation constant, obtained by the cold (beam-absent) field analysis, was fed back into the small-signal gain equation of a gyro-traveling wave tube (TWT) to assess the technique of tapering the structure cross section for widening the bandwidth of the device, a technique that is known to broadband the device in...
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Phase stability of a W-band TE01 gyro-traveling-wave-tube (Gyro-TWT) with distributed loss and tapered magnetic field is studied in this paper. The phase sensitivity of electron beam voltage, current, and magnetic field is analyzed by using nonlinear theory. The influences on phase stability of important factors such as distributed loss, tapered magnetic field, and input frequency are p...
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In this paper, the behavior of a gyro-traveling wave tube (gyro-TWT) amplifier, in which an axis-encircling electron beam synchronously interacts with the azimuthal modes of the vane-loaded cylindrical waveguide interaction structure, has been analyzed. The dispersion characteristics and gain-frequency response of the nearby waveguide modes (TE01, TE21, and TE41) f...
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The frequency-tunability for a Smith-Purcell free-electron laser oscillator in the millimeter wave region is enhanced with the aid of a quasi-periodic distributed Bragg reflector (DBR) made of metallic waveguide gratings with rectangular grooves. For a quasi-periodic DBR, the grating parameters are adjusted by means of genetic algorithms to get the desired reflection coefficient, thus compensating...
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In this paper, the finite-size vane (FSV) sheath-helix model is proposed to calculate the cold-test characteristics of T-shaped vane loaded helical slow-wave structures (SWSs), in which the effects of the finite number and dimensions of the T-shaped vanes are taken into account, and also the angular harmonics generated due to the angular periodicity of the vanes are considered. Based on this model...
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The development of an S-band klystron with bandwidth of more than 11% is introduced in this paper. At the peak power level of 800 kW, the efficiency of more than 30%, the gain of more than 40 dB, the equal-driving (constant input power across the bandwidth) relative instantaneous bandwidth of 11.2%, the average power of larger than 8.8 kW, and the power fluctuation within bandwidth of less than 1....
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We present a general discussion of multiple-gap cavity characteristics and stability issues along with the detailed design of a five-cavity, broadband, high-power multiple-beam klystron (MBK). The predicted performance characteristics of this S-band fundamental-mode klystron circuit include a 3-dB bandwidth of 6.7% with peak power ~600 kW and an electronic gain of 33 dB. The circuit is powered by ...
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Two-dimensional steady-state analysis for the electron flow in an inward-emitting coaxial virtual cathode oscillator is presented. Based on the relativistic-fluid-Maxwell equations, the relativistic plasma frequency characteristics and its dependence on the geometric parameters are derived. The resulting plasma frequency solution scales linearly with the diode current. The exact dominant frequency...
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Simulations results of a relativistic magnetron using the MAGIC particle-In-cell (PIC) code are reported. The time evolution of the buildup process for different magnetron azimuthal modes is studied for a variety of initial voltages, with and without a second, oscillatory small voltage in addition to the drive. The self-field is seen to evolve so that the anode-cathode voltage nears the Buneman-Ha...
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The output characteristics of magnetrons can be significantly improved by using a cathode that is transparent to the synchronous electromagnetic fields thereby providing improved conditions for fast conversion of the electrons' potential energy into electromagnetic energy. The transparent cathode consists of separate longitudinal metal strips, arranged to form a cylindrical surface, that act as in...
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We demonstrate the ability to form simple radiation patterns from a relativistic magnetron with axial extraction. This is achieved by tapering onto a conical antenna only those cavities of the anode block that correspond to the symmetry of the radiated modes. The efficiency of mode conversion of the operating pi-mode into a radiated mode using this method is demonstrated using computer simulations...
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Rapid startup, increased pulsewidth, and mode locking of magnetrons have been explored experimentally on a relativistic magnetron by radio frequency (RF) priming. Experiments utilize a -300 kV, 2-8 kA, 300-500-ns electron beam to drive a Titan six-vane relativistic magnetron (5-100 MW output power in each of the three waveguides). The RF priming source is a 100-kW pulsed magnetron operating at 1.2...
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Quasi-optical launcher designs for high-power gyrotrons use perturbations of the waveguide surface following the cavity to convert the high order cavity mode to a Gaussian-like beam, which is then radiated from a spiral cut of the waveguide. The design technique of these launchers is based on an analytic theory that specifies the mode amplitudes to obtain a Gaussian-like distribution of the field ...
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An experimental test facility has been established for measuring the secondary electron yield (SEY) of materials thought to be suitable for low yield vacuum electronic applications such as collectors in high-power microwave (HPM) tubes. Experiments can be broadly divided into two energy-regimes: a high-energy (1-50 keV) and a low-energy (10 eV-1 keV) regime. Measurements of SEY at high energies ar...
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Due to the requirement of a newly designed ultra-broadband chemical vapour deposition (CVD)-diamond window for a multifrequency 1-MW short-pulse gyrotron at Forschungszentrum Karlsruhe (FZK), the existing beam-forming mirror system inside the gyrotron tube has to be modified. This paper describes the design scheme for shaping the output beams from the launching antenna waveguide into the desired f...
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Optimized designs for wideband reflector gratings in rectangular waveguide are presented for application in microwave electron beam devices. A moment method procedure for a general dual-plane step in rectangular waveguide and a generalized scattering matrix approach are used to analyze the reflector gratings. A least squares optimization method is used to determine optimized dimensions for reflect...
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